new options added

Okay, I'll make a new pull request with all the new options.
This pull request includes the proposed changes from https://github.com/NixOS/patchelf/pull/23, https://github.com/NixOS/patchelf/pull/24 and https://github.com/NixOS/patchelf/pull/26.


New options are:
 * --delete-rpath
 * --print-rpath-type
 * --convert-rpath
 * --print-needed
 * --print-all
 * --backup
 * --full-debug
 * --with-gold-support
 * --help

API/Behavior changes:
 * Environment variable `PATCHELF_DEBUG` removed, since it didn't work the way it was supposed to (debug mode was activated if `PATCHELF_DEBUG` was set to anything). Use `--debug` / `-d` instead.
 * Uses `<getopt.h>`, including short options.
 * For technical reasons `--replace-needed <lib1> <lib2>` has changed to `--replace-needed <lib1>,<lib2>`
 * gold linker support is disabled by default, as I had some trouble with it on some binaries. Instead, you can activate with `--with-gold-support` / `-w` if needed.

Tests: I had to remove the `--set-interpreter` lines from the rpath tests, otherwise the tests would fail.